mcg vs mg, mL and units: the four numbers that trip people up

mcg vs mg comes down to a factor of one thousand: 1 mg is 1,000 mcg. mL and units are a different kind of number altogether, because they measure liquid rather than peptide.

All four appear in almost every peptide calculation, and none of them is complicated on its own. The trouble is that two of them measure a completely different kind of thing from the other two, and the notation does not make that obvious.

Inside each family, conversion is a fixed multiplication. Between families, it is not possible at all without knowing the concentration.

How many mcg are in 1 mg?

One milligram is 1,000 micrograms. To turn mcg into mg, divide by 1,000, so 250 mcg is 0.25 mg.

Vials are commonly labelled in mg while doses are commonly written in mcg, so this conversion sits in the middle of nearly every calculation. Being off by a factor of 1,000 does not look like a small error, which is at least a mercy.

Is 5 mcg the same as 5 mg?

No. 5 mg is a thousand times more than 5 mcg, because both units measure mass and a milligram is the larger one.

The same goes for asking which is stronger, mcg or mg. Neither is a strength; they are sizes. A number only means something alongside its unit, which is why a figure written without one is worth rechecking before it goes into a calculation.

mL and units: a factor of one hundred

A U-100 insulin syringe is graduated at 100 units per millilitre. So one unit is 0.01 mL, and the conversion holds regardless of what is in the barrel, because the markings describe volume, not content.

The U-100 part matters. U-40 and U-500 syringes are graduated on a different scale, and applying a U-100 conversion to them produces a wrong answer that looks entirely plausible. The scale is printed on the barrel, and the insulin syringe unit converter reads U-40 as well as U-100.

How do you convert mg to mL?

Divide the dose in mg by the concentration of the vial in mg/mL. Without a concentration there is no answer, because mg measures peptide and mL measures liquid.

Concentration is how much peptide is in each millilitre, and it comes from the reconstitution, which the bacteriostatic water calculator lets you try before any water goes in:

Concentration (mg/mL) = vial strength (mg) ÷ diluent volume (mL)

This is why a dose in mcg has no fixed unit equivalent. 250 mcg is 5 units from one vial and 15 units from another, depending entirely on how each was reconstituted. Anyone who quotes a unit figure without stating the concentration has left out the load-bearing half of the sentence.

How to get from mcg to syringe units

Five steps take a dose in mcg to a mark on a U-100 syringe. Only the first number comes from outside the arithmetic: the dose your healthcare professional has set.

1. 1
   Start with the dose your healthcare professional has set

   Usually in mcg, and worth confirming whether it is mcg or mg. This is the only number in the chain that is not arithmetic.

2. 2
   Convert mcg to mg

   Divide by 1,000. 250 mcg becomes 0.25 mg.

3. 3
   Find the concentration

   Vial strength divided by diluent volume. A 5 mg vial in 2 mL is 2.5 mg/mL.

4. 4
   Convert mg to mL

   Dose in mg divided by concentration. 0.25 ÷ 2.5 = 0.1 mL.

5. 5
   Convert mL to units

   Multiply by 100. 0.1 mL is 10 units on a U-100 barrel.

How do you check the answer?

Check that the draw fits the barrel and that the dose count fits the vial. A 5 mg vial at 250 mcg a dose holds 20 doses, and 20,000 means an mg/mcg slip.

On the barrel: a calculated draw of 200 units does not fit in a 1 mL syringe, so something upstream is wrong. On the vial: vial strength divided by dose should give a whole-ish number of doses. 5,000 mcg ÷ 250 mcg is 20, which is plausible. If that arithmetic returns 20,000, or 0.02, an mg/mcg conversion went the wrong way.